This invention relates to phase shifters and more particularly to a phase shifter using a metal-insulator-semiconductor (MIS) type device. Diode phase shifters using PIN diodes as the switching element have exhibited excellent performance at microwave frequencies. The main drawback is the high current (several milliamps) required by PIN diodes in their forward bias state. A phased array antenna system comprising many such phase shifters requires several amperes of bias current, resulting in complex and costly drivers and bias distribution circuitry. Reverse bias junction varactors have been used in reflection type phase shifters, overcoming the current requirement problem. The voltage variable capacitance becomes a voltage variable reflection coefficient angle when terminating in a transmission line. In a reflection type phase shifter, this is translated into a voltage variable transmission coefficient angle. It can be seen that as the bias changes, the phase shift changes and, because the device is reverse biased at all times, the steady state current required is very small, on the order of microamperes or nanoamperes.
The reverse bias junction varactor approach suffers limitations, however, due to the shape of the C-V curve of a standard varactor. At one relatively high reverse bias level, the capacitance is approximately constant with voltage. However, at a lower reverse bias level, the capacitance changes relatively rapidly with voltage. If the RF voltage swing is large, a nonlinear waveform is generated with high harmonics. Moreover, as the nonlinear portion of the curve is approached, the average capacitance changes causing the phase to change. The phase shift is therefore not constant with power level. Another problem is that power supply drift or ripple will cause phase error. This is sometimes turned into an advantage since it enables phase trimming through supply level adjustment, but this is not usually a desirable way to trim as the disadvantages frequently outweigh the advantages.